Human immunodeficiency virus (HIV) breaks down host defenses, by infecting cells of the immune system that orchestrate anti-viral immunity. Among these cell types, dendritic cells (DCs) perform critical functions in the initiation, and regulation of immune responses. DCs are the quintessential antigen-presenting cells (APC), and are strategically located in tissues, where they can sense invading pathogens, and convey pathogen-related information to lymphocytes, thus initiating adaptive immunity, and immune memory. However, during HIV infection, the host is able to mount only a weak and transient immune response, resulting in the establishment of chronic infection. Non-human primate models of infection have revealed that "prime-boost" vaccine strategies that utilize live viral vectors are capable of inducing T cell immunity, and controlling viral replication. However, the use of live viral vectors raises concerns about potential adverse effects. Therefore, there is an urgent need to devise novel and safe strategies to augment HIV-immunity. In this grant proposal we will test a strategy, which enhances the numbers and functions of DCs, in vivo to elicit robust anti-viral immunity in non-human primates, in both prophylactic and therapeutic vaccine settings. To achieve this goal, we propose to exploit the remarkable abilities of a DC growth factor, Fit-3 ligand (FL), and a DC activator (CpG DNA). FL is known to expand DC numbers in both mice and humans. CpG DNA triggers toll-like receptor-9 (TLR-9) expressed in plasmacytoid DC, and induces the production of copious amounts of the antiviral cytokine, interferon (IFN alpha). We hypothesize that the combination of both FL and CpG DNA in the context of DNA vaccines will elicit enhanced levels of anti-viral immune responses in both prophylactic and therapeutic settings. These issues will be addressed in the following specific aims: Aim 1: To determine the parameters for optimal DC expansion and activation, [with FL+CpG], necessary to elicit robust antigen-specific immunity in healthy macaques. Aim 2: To determine whether FL-induced DC expansion, followed by CpG-DNA induced DC activation in vivo, enhances prophylactic vaccine immunity to SIV in macaques. Aim 3: To determine whether FL-induced DC expansion, followed by CpG-DNA induced DC activation in vivo, enhances therapeutic vaccine immunity in macaques chronically infected with SIV. If successful, such a strategy will likely greatly advance the design of effective vaccines against HIV/AIDS in humans in the future. [unreadable] [unreadable]